In order to provide large bandwidth and high-capacity coverage, Access Nodes (ANs) in a network typically share the entire spectrum available in the network. For example, in a millimeter wave (mmW) network, a number of “high-capacity coverage islands” are deployed and the entire spectrum available in the mmW network is reused by each of these islands.
FIG. 1 shows an exemplary scenario of spectrum reuse. Two networks, a network 110 and a network 120, are shown in FIG. 1. The coverage area of the network 110 and the coverage area of the network 120 partially overlap each other, as shown by the dashed lines in FIG. 1. The networks 110 and 120 may belong to different network operators and may be allocated with the same spectrum. The network 110 includes two ANs, AN 111 and AN 112, and the network 120 includes two ANs, AN 121 and AN 122. All the ANs 111, 112, 121 and 122 in FIG. 1 share the same spectrum. FIG. 1 also shows three User Equipments (UEs) 101, 102 and 103. Here, the UE 101 is served by the AN 111, the UE 102 is served by the AN 112 and the UE 103 is served by the AN 122. It can also be seen from FIG. 1 that the UE 102 is located in the overlapped area. In the following, communications between the UE 102 and the AN 112 will be described as an example, without loss of generality.
In order for the UE 102 to communicate with the AN 112, the AN 112 broadcasts beacons containing necessary information periodically within its coverage.
FIG. 2 shows an exemplary frame structure for communication between the UE 102 and the AN 112. The AN 112 broadcasts beacons at a beacon period, which may include a number of frames. As shown in FIG. 2, each beacon is transmitted in the first time slot (referred to as “beacon slot” hereinafter) in the first frame of the beacon period. It can be appreciated by those skilled in the art that it is illustrative only and the beacon slot can be placed at any other location as appropriate. The other slots can be used for uplink (UL) and/or downlink (DL) transmissions.
A beacon contains a sync signal sequence (such as Primary Synchronization Signal (PSS) or Secondary Synchronization Signal (SSS) in Long Term Evolution (LTE) system), a reference signal (RS) sequence and system information. The system information includes a Network Identity (NI) which globally uniquely identifies the network the AN 112 belongs to (i.e., the network 110). In this context, a “network” can be a Public Land Mobile Network (PLMN) and different “networks” are typically managed by different network operators. The system information further includes an AN Identity (AI) which uniquely identifies the AN 112 locally within the network 110. The system information further includes a Physical AN Identity (PANI) associated with physical layer functions of the AN 112. A PANI is uniquely associated with a combination of sync signal sequence and reference signal sequence (and their time and frequency locations) and such association is common among different networks.
In operation, the UE 102 detects the sync signal sequence blindly when scanning or listening to a beacon channel. When the UE 102 successfully detects the sync signal sequence, it knows the PANI from the detected sync signal sequence and thus determines the reference signal sequence based on PANI. Then, the UE 102 derives a channel estimation based on the determined reference signal sequence, and finally decodes the system information and the subsequent data transmissions based on the channel estimation.
However, since the same spectrum is reused by the network 110 and the network 120 (and thus the AN 112 and the AN 121, for example), the UE 102 also suffers from a problem of “PANI collision”. There are typically a limited number of PANIs available (i.e., 504 in LTE) and thus different ANs in the same or different network(s) may have the same PANI. With cell planning, neighboring ANs of the same network can be allocated with different PANIs by the network operator. However, ANs from different networks (e.g., AN 112 and AN 121) may have the same PANI due to lack of inter-operator coordination.
FIG. 3 shows an exemplary scenario of “PANI collision”. The UE 102 may receive two beacons simultaneously, one from the AN 112 and the other from the AN 121 (it is assumed here that the AN 112 and the AN 121 are synchronized and thus the beacons are aligned with each other). It is assumed here that the AN 112 and the AN 121 are allocated with the same PANI. That is, the beacons from the AN 112 and the AN 121 contain the same sync signal sequence and the same reference signal sequence. The UE 102 cannot realize that it is receiving beacons from different ANs. Hence, it will successfully detect the sync signal sequence and accordingly derive a combined channel estimation of a channel between the UE 102 and the AN 112 and a channel between the UE 102 and the AN 121. With such combined channel estimation, the UE 102 cannot decode the system information in either of the beacons (hence it will never know that the beacons are transmitted from different ANs) and cannot decode any subsequent data transmission from the AN 112.
In addition to the above “PANI collision” problem, the UE 102 may also suffer from intra-frequency interference from the AN 111 and/or the AN 121. Such intra-frequency interference on the beacon transmission will result in a degraded Signal to Interference Ratio (SIR) of the beacon received at the UE, which in turn reduces the probability that the UE 102 can successfully decode the beacon and establish a connection with the AN 112.
There is thus a need for an improved solution for beacon transmission.